1. Field of the Invention
This invention relates to digital image enhancement.
2. Description of the Prior Art
Image enhancement is often used to improve the subjective portrayal of detail in a video or still image. Here, however, and throughout this specification, the term "enhance" simply means to apply various processing techniques to change the portrayal of detail or the relative portrayal of different spatial frequencies within the image. It does not necessarily imply any absolute or even any other improvement in the image, although often the aim is to provide at least a subjective improvement.
A basic previously proposed technique for electronic image enhancement is first to separate out certain spatial frequency bands in an input image signal using, for example, one or more high-pass or band-pass filters, in order to generate a "detail" signal containing frequency components of the image considered to be most representative of image detail. The detail signal is then scaled and added to the original image signal. The result is that the level of the frequency components considered to be most representative of image detail is increased in the output, enhanced, image with respect to other frequency components of the image.
An example apparatus of this type is schematically illustrated in FIG. 1 of the accompanying drawings, in which an input video signal is supplied in parallel to a band-pass filter and an adder 20 (possibly via a compensating delay device--not shown). The band-pass filter filters certain frequency bands of the video signal to generate a detail signal. The magnitude of the detail signal is then adjusted at a multiplier 30, by multiplying the detail signal by a "detail gain" coefficient. The gain-adjusted detail signal is then added to the original input video signal at the adder 20, and finally the resulting "enhanced" signal is supplied to a clipping circuit 40 where the level of the enhanced signal is clipped if it goes beyond predefined upper or lower limits on signal level.
A problem lies in the final stage of clipping of the enhanced image signal. This clipping is necessary in the apparatus of FIG. 1 because the addition of the detail signal to the original image signal can increase the amplitude of the resulting enhanced image signal beyond allowable limits--e.g. beyond a predetermined peak white level or below a predetermined black level. However, the clipping operation is a highly non-linear process and so can introduce distortions to the image signal. In a digital (sampled) system, the clipping process can also introduce frequency harmonics above the Nyquist limit; when the signal is finally converted back to analogue form (e.g. for display) these harmonics can give rise to subjectively disturbing alias distortions within the image.
In an attempt to avoid the harmonics and alias distortions introduced by the clipping process, it has been proposed that the input sampled image signal is up-converted or super-sampled before enhancement, to be sub-sampled back to the original sampling rate afterwards. An example of an apparatus for carrying out this process is shown schematically in FIG. 2 of the accompanying drawings.
In FIG. 2, the input video signal is supplied first to an interpolator or sample rate up-converter 50. This generates an intermediate "super-sampled" video signal having a higher sample rate than the input video signal. Importantly, this increase in sample rate also means that the Nyquist limit for the intermediate video signal is higher than that for the input video signal.
The intermediate video signal is then processed in a similar manner to that shown in FIG. 1, whereby it is band-pass filtered 10', the resulting detail signal is multiplied 30' by a gain coefficient, and the gain-controlled detail signal is added 20' to the intermediate video signal, with the resulting signal being clipped 40'.
A final step is that the output of the clipping circuit 40' is supplied to a decimating or sample rate down-converting filter 60, where the higher sample rate of the output signal from the clipping circuit 40' is down-converted to the sample rate of the original input video signal.
However, the disadvantage of super-sampling the video signal is that the super-sampling filter (interpolator) and the subsequent downsampling or decimation filter affect the frequency response of the image signal whether or not enhancement is applied. This can be alleviated by making the filters very large, i.e. using a very large number of filter taps, but this has the further disadvantages of adding a significant delay to the image signal (important in the case of video signal enhancement) and that the filters then require a very large amount of memory.